Generally, when concrete structures such as the platform of a railway station, the base of subway and the pavements of roads, parking lots, runways, taxiways, and aprons of airports are built, a mixture of cement, sands, pebbles and water is charged. During the process of curing of the concrete, the water content within the concrete is evaporated to form tiny cavities within the concrete. The concrete having such cavities is subjected to expansions and contractions by the atmospheric heat, and this phenomenon is repeated to finally produce cracks in the concrete slab.
When cracks are formed in the concrete, water intrudes into the cracks to produce various adverse effects. Considering the fact that the formation of the cracks is more frequent proportionately to the total surface area of the concrete formed, the method of reducing the surface area of concrete is devised.
The method of reducing the total surface area of the concrete is such that the total surface area is divided into partitions by using elongate wooden bars.
When the total area is partitioned in this way, the small areas thus partitioned produced only small amount of cracks to such extent that the concrete structure is not substantially affected, thereby achieving the intended purpose.
However, the wooden bars (to be called partitioning member below) which are used for partitioning the total area of the concrete surface can be produced in a uniform thickness and in a uniform height. Therefore they could be used only in horizontal concrete slabs.
That is, referring to FIGS. 1A and 1B, the ceiling of a structure includes: a concrete layer 10 forming a ceiling; a polyethylene foam layer 11 installed upon the concrete layer 10 for heat-insulating and for preventing the intrusion of water; another concrete layer 12 formed upon the polyethylene foam layer 11; a waterproof layer 13 composed of tar and for preventing the intrusion of water; another polyethylene foam layer 14 installed upon the waterproof layer 13; and the final slab 15 formed upon the layer 14.
In the above described slab 15, a flexible partitioning member 16 is installed for preventing cracks. This flexible partitioning member 16 has a uniform thickness and a uniform height as described above, and therefore, it is disposed horizontally upon the upper polyethylene foam layer 14. Therefore it has to be installed horizontally. Thus, if the slab 15 is formed horizontally, water does not flow down but is stagnated.
Therefore, the slab 15 which is the uppermost layer of the structure is formed in an inclined form as shown in FIG. 1B. However, the flexible partitioning member 16 has a uniform height, and therefore, the flexible partitioning member 16 is also inclined as much as the inclination angle. The lower side of this inclined flexible partitioning member is separated from the upper face of the polyethylene foam 14. Therefore, the concrete which is injected into the respective partitions is connected through the separated gaps of the flexible partitioning member 16, with the result that the total concrete portions are connected together, thereby producing cracks in the slab 15.
In the above described ceiling structure, the upper polyethylene foam 14 can be omitted in accordance with the design specification, and the slab 15 is directly formed upon the waterproof layer 13.
The conventional flexible partitioning member 16 as described above has a uniform height. Therefore, in the case where the slab is formed in an inclined form, the lower side of the flexible partitioning member leaves a gap, so that the concrete which is injected into the respective partitions is connected together, thereby producing cracks.
Further, in the case of the conventional flexible partitioning member 16, the edge of the leading end of the flexible partitioning member 16 and the edge of the leading end of the slab 15 are disposed on the same line. In this state, when the concrete slab within a partition is contracted, the portion of the concrete slab which is contacted with the flexible partitioning member 16, i.e., the portion A, forms a slit through which rainwater intrudes. Such intruded water is stagnated below the flexible partitioning member 16, and the water is freezed in winter seasons to expand its bulk. Due to the expansion of the bulk of the freezed water, cracks are produced in the concrete.